A device that has a plurality of semiconductor modules with built-in switching elements such as an IGBT element, and a pair of DC (direct current) bus bars (a positive bus bar and a negative bus bar) is known as an electric power converter that converts power between a DC power and AC power, for example (refer to Japanese Patent Application Laid-Open Publication No. 2009-240037). In this electric power converter, the DC power from DC power supply is supplied to the semiconductor module via the pair of DC bus bars. Then, by switching on or off the switching element, the DC power is converted to AC power, and an AC load is configured to be driven by using the obtained AC power.
When the switching element is switched on or off, a noise current is generated. This noise current may be transmitted to an external device such as the DC power supply through the DC bus bars if not sufficiently removed. Therefore, efforts to sufficiently remove the noise current have been studied.
For example, a bypass capacitor is disposed in the electric power converter, and the DC bus bars and ground are connected through the bypass capacitor. Accordingly, the noise current generated from the switching element flows through the DC bus bars, and flows to ground through the bypass capacitor. Therefore, it is possible to suppress the problem of the noise current being transmitted to the external device.
However, even if the bypass capacitor is disposed, there is a problem that a frequency band of the noise current that can be removed is narrow. In other words, a relatively large parasitic inductance L exists to the switching element that is a source of the noise current and the DC bus bar or the like that connects the bypass capacitor and the switching element. Assuming that a capacitance of the bypass capacitor is C and the frequency of the noise current is f, an impedance Z from the switching element to ground is represented by the following formula.Z=2nfL+1/(2nfC)
As can be seen from this formula, when the parasitic inductance L is large, it is possible to reduce a term 2nfL when the frequency f of the noise current is low. Moreover, even when the frequency f is low, a term 1/(2nfC) can also be reduced if the capacitance C of the bypass capacitor is sufficiently increased. Therefore, for noise current with a low frequency f, it is possible to reduce the overall impedance Z, and the noise current can flow to ground through the bypass capacitor.
However, when the frequency f of the noise current becomes high, the overall impedance Z increases because the term of 2nfL increases. Therefore, noise current with a high frequency f hardly flows to ground. Thus, an electric power converter that can remove not only a noise current with a low frequency f, but can remove a noise current with a high frequency, that is, an electric power converter capable of removing a noise current of a wider frequency band is desired.